CASEIN KINASE-II IS REQUIRED FOR CELL-CYCLE PROGRESSION DURING G(1) AND G(2)/M IN SACCHAROMYCES-CEREVISIAE

The catalytic subunit of Saccharomyces cerevisiae casein kinase II (Sc CKII) is encoded by the CKA1 and CKA2 genes, which together are essential for viability. Five independent temperature-sensitive alleles of the CKA2 gene were isolated and used to analyze the function of CHII during the cell cycle. Following a shift to the nonpermissive temperature, cka2(ts) strains arrested within a single cell cycle and exhibited a dual arrest phenotype consisting of 50% unbudded and 50% large-budded cells. The unbudded half of the arrested population contained a single nucleus and a single focus of microtubule staining, consistent with arrest in G(1). Most of the large-budded fraction contained segregated chromatin and an extended spindle, indicative of arrest in anaphase, though a fraction contained an undivided nucleus with a short thick intranuclear spindle, indicative of arrest in G(2) and/or metaphase. Flow cytometry of pheromone-synchronized cells confirmed that CKII is required in G(1), at a point which must lie at or beyond Start but prior to DNA synthesis. Similar analysis of hydroxyurea-synchronized cells indicated that CKII is not required for completion of previously initiated DNA replication but confirmed that the enzyme is again required for cell cycle progression in G(2) and/or mitosis. These results establish a role for CKII in regulation and/or execution of the eukaryotic cell cycle.